Communication dipole for implantable medical device

ABSTRACT

This disclosure is directed to an implantable medical device having a communication dipole configured in accordance with the techniques described herein. In one example, the disclosure is directed to an implantable medical device comprising a housing that encloses at least a communication module, a first electrode of a communication dipole electrically coupled to the communication module and an electrically conductive fixation mechanism that is electrically coupled to a portion of the housing and wherein a portion of the fixation mechanism is configured to function as at least part of a second electrode of the communication dipole. The electrically conductive fixation mechanism includes a dielectric material that covers at least part of a surface of the fixation mechanism. The communication module is configured to transmit or receive a modulated signal between the first electrode and second electrode of the communication dipole.

This application claims the benefit of U.S. Provisional Application Ser.No. 61/437,198, filed on Jan. 28, 2011, the content of which isincorporated herein by reference in its entirety.

TECHNICAL FIELD

The disclosure relates generally to implantable medical devices and, inparticular, to a communication dipole for implantable medical devices.

BACKGROUND

A wide variety of implantable medical devices (IMDs) that sense one ormore parameters of a patient, deliver a therapy to the patient, or bothhave been clinically implanted or proposed for clinical implantation inpatients. An IMD may deliver therapy to or monitor a physiological orbiological condition with respect to a variety of organs, nerves,muscles, tissues or vasculatures of the patient, such as the heart,brain, stomach, spinal cord, pelvic floor, or the like. The therapyprovided by the IMD may include electrical stimulation therapy, drugdelivery therapy or the like.

The IMD may exchange communications with another device. The IMD mayexchange communications with another device that is implanted, attachedto (e.g., worn by) the patient or otherwise located near the patient.The information exchanged may be information related to a condition ofthe patient, such as physiological signals measured by one or moresensors, or information related to a therapy delivered to the patient.The IMD may also receive information from the other device, such asinformation that may be used to control or configure a therapy to beprovided to the patient. The IMD and the other device may exchangeinformation using any of a variety of communication techniques,including inductive telemetry, magnetic telemetry, radio frequency (RF)telemetry or the like.

SUMMARY

Intra-body communication is one communication scheme that may be used tocommunicate information to and from an implantable medical device.Intra-body communication uses the body of the patient as thecommunication channel. The human body has dielectric properties thatallow the body to act as a transmission medium for electrical currents.Thus, intra-body wireless communication exploits the transmissionchannel of electrolytic-galvanic coupling with the device electrodes andthe ion medium (or other properties) of cellular fluids of the patient.A transmitter of either an IMD or an external device applies a modulatedelectrical current between a pair of electrodes to transmit a modulatedsignal. A pair of electrodes of the receiving device, which are also incontact with the body of the patient, receive the modulated signal as anelectric potential difference across a pair of electrodes of thereceiving device. The pair of electrodes of either the transmittingdevice or the receiving device may be referred to herein as a“communication dipole,” a “transmit dipole,” or a “receive dipole” dueto its operation resembling that of a dipole antenna.

Due to the small size of IMDs, and especially devices configured forimplantation within the vasculature of the patient, the distance betweenelectrodes forming the communication dipole is typically limited.Electrodes used for intra-body communication may, for example, typicallybe placed at opposite ends of a housing of the IMD. It is desirable,however, to increase the distance between the electrodes of thecommunication dipole to increase the strength of the communicationsignal transmitted and received via intra-body communication. Inaccordance with the techniques of one aspect of this disclosure, the IMDis configured to utilize a portion of a fixation mechanism of the IMD aspart of one or both of the electrodes of the communication dipole,thereby increasing the distance separating the electrodes. In oneexample, a portion of the fixation mechanism is utilized as at least apart of a first of the dipole electrodes and the other dipole electrodemay be formed on or integrated in the housing of the IMD. In someinstances, the portion of the fixation mechanism and a second portion ofthe housing may be mechanically and/or electrically coupled to functionas the first of the dipole electrodes.

Additionally, the location of the portion of the fixation mechanismforming part of the first of the dipole electrodes may be selected toprovide at least some control of the orientation of the transmittingdipole when the IMD is rotated, e.g., around a central axis of avasculature during implantation. In instances in which the communicationdipole is formed from electrodes placed at opposite ends of a relativelycylindrical housing of the IMD there is very little control over theorientation of the communication dipole. However, selecting a portion offixation mechanism that functions as part of the first electrode of thecommunication dipole such that the orientation of the axis of thecommunication dipole (axis defined between the portion of the housingconfigured to function as the second electrode of the communicationdipole and the portion of fixation mechanism configured to function aspart of the first electrode of the communication dipole) is offset fromthe central axis of the vasculature when the IMD is positioned withinthe vasculature provides the capability to have more control of theorientation of the dipole. In this case, rotation of the IMD within thevasculature of the patient may change the directionality at which thecurrent is radiated by the transmitting communication dipole.

In one example, the disclosure is directed to an implantable medicaldevice comprising a housing that encloses at least a communicationmodule, a first electrode of a communication dipole electrically coupledto the communication module and an electrically conductive fixationmechanism that is electrically coupled to a portion of the housing andwherein a portion of the fixation mechanism is configured to function asat least part of a second electrode of the communication dipole. Theelectrically conductive fixation mechanism includes a dielectricmaterial that covers at least part of a surface of the fixationmechanism. The communication module is configured to transmit or receivea modulated signal between the first electrode and second electrode ofthe communication dipole.

This summary is intended to provide an overview of the subject matterdescribed in this disclosure. It is not intended to provide an exclusiveor exhaustive explanation of the techniques as described in detailwithin the accompanying drawings and description below. Further detailsof one or more examples are set forth in the accompanying drawings andthe description below. Other features, objects, and advantages will beapparent from the description and drawings, and from the statementsprovided below.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a conceptual diagram illustrating an example medical system.

FIG. 1B is a conceptual diagram illustrating another example medicalsystem.

FIG. 2 is a conceptual diagram illustrating an IMD implanted in a heartof a patient.

FIGS. 3A-3D are schematic diagrams illustrating enlarged views of theIMD of FIG. 2 from various viewpoints.

FIG. 3E is a schematic diagram illustrating another example IMD.

FIG. 3F is a schematic diagram illustrating another example IMD.

FIGS. 4A and 4B are schematic diagrams illustrating other example IMDs.

FIG. 5A-5C are schematic diagrams illustrating other example IMDs.

FIGS. 6A-6C are graphs illustrating an example plot of effective dipolelength and impedance versus the amount of fixation mechanism that isexposed.

FIGS. 7A and 7B are a schematic diagram illustrating other example IMDs.

FIGS. 8A-8C are schematic diagrams illustrating a further example of anIMD.

FIG. 9 is a functional block diagram illustrating components of animplantable medical device in further detail.

DETAILED DESCRIPTION

FIG. 1A is a conceptual diagram illustrating an example medical system10. Medical system 10 includes an implantable medical device (IMD) 14and an external device 16. Medical system 10 may, however, include moreof fewer implanted or external devices.

IMD 14 may be any of a variety of medical devices that sense one or moreparameters of patient 12, provide therapy to patient 12 or a combinationthereof In one example, IMD 14 may be a leadless IMD. In other words,IMD 14 is implanted at a targeted site with no leads extending from IMD14, thus avoiding limitations associated with lead-based devices.Instead, sensing and/or therapy delivery components are integratedwithin IMD 14. In the case of a leadless sensor, IMD 14 includes one ormore sensors that measure the physiological parameter(s) of patient 12.In one example, IMD 14 may comprise an implantable device incorporatinga pressure sensor that is placed within a vasculature or chamber of aheart of patient 12.

IMD 14 may, in some instances, provide therapy to patient 12. IMD 14 mayprovide the therapy to patient 12 as a function of sensed parametersmeasured by the sensor of IMD 14 or sensed parameters received fromanother device, such as another IMD or a body worn device. As oneexample, IMD 14 may be a leadless cardiac IMD that provides electricalstimulation therapy (e.g., pacing, cardioversion, defibrillation, and/orcardiac resynchronization therapy) to the heart of patient 12 via one ormore electrodes as a function of sensed parameters associated with theheart. In yet a further example, IMD 14 may provide therapy to patient12 that is not provided as a function of the sensed parameters, such asin the context of neurostimulation. Although described above in thecontext of electrical stimulation therapy, IMD 14 may provide othertherapies to patient 12, such as delivery of a drug or other therapeuticagent to patient 12 to reduce or eliminate the condition of the patientand/or one or more symptoms of the condition of the patient, or provideno therapy at all.

External device 16 communicates with IMD 14 using intra-bodycommunication. Intra-body communication as used herein refers to a datatransmission scheme that uses the human body as the communicationchannel. The human body has dielectric and conductive properties thatallow the body to act as a transmission medium for electrical currents.Intra-body wireless communication exploits these properties of bodytissue and fluid as a transmission channel. In some instances, theintra-body wireless communication scheme may exploit the transmissionchannel of electrolytic-galvanic coupling with the device electrodes andthe ion medium (or other properties) of extra and intracellular fluidsof patient 12. External device 16 and IMD 14 may communicate usingintra-body communication over frequencies ranging from a few kilohertzto a few megahertz. Higher frequency communication signals may be usedto increase data transmission rates.

IMD 14 and external device 16 each include respective electrodes 18 usedfor intra-body communication. Electrodes 18 a and 18 b of IMD 14 form acommunication dipole that may be used as both a receive dipole and atransmit dipole of IMD 14. Electrodes 18 c and 18 d of external device16 form a communication dipole that may be used as both a receive dipoleand a transmit dipole of external device 16, each for use in intra-bodycommunication. A transmit dipole of either IMD 14 or external device 16injects modulated electrical current between the pair of electrodesforming the transmit dipole, which introduces a modulated current intothe body of patient 12. In general, the modulated current is radiated ina generally toroid shape about the axis of the dipole. The axis of thedipole may be defined by an imaginary line extending from electrode 18 ato electrode 18 b. If r >>d, where r is the distance between theobservation point and the center of the dipole and d equals the transmitdipole length, then the potential θat a point in space can beapproximated by the equation:

${\varnothing = {\frac{I_{0}.d}{4{\pi\sigma}\; r^{2}}\cos\;\theta}},$where I_(o) is the dipole current, σ is the conductivity of thesurrounding medium, and θ is the polar angle. The polar angle generallyrefers to the angle between the line depicting the axis of the dipoleand the line drawn from the center of the dipole to one of the receiveelectrode positions. The value of d corresponds to the distance betweenthe electrodes of the transmitting dipole, which is generally on theorder of millimeters to centimeters for IMD 14.

A receive dipole of the other one of IMD 14 or external device 16, alsoin contact with the body of patient 12, receives the modulated signal asan electric potential difference across the pair of electrodes which arealso in contact with the body of patient 12. Electrodes 18 a and 18 b ofIMD 14 and the electrodes 18 c and 18 d of external device 16 can eachbe configured to function as either the transmit dipole or the receivedipole. Electrodes 18 c and 18 d of external device 16 may be arrangedon the body of patient 12 such that they are oriented in a manner toprovide sufficient coupling to the communication dipole of IMD 14. Forexample, an optimal or at least acceptable orientation may be determinedat time of implantation and provided to patient 12 such that patient 12can place external electrodes 18 c and 18 d in appropriate positions toenable intra-body communication with IMD 14.

External device 16 may communicate with IMD 14 via intra-bodycommunication to retrieve information from IMD 14, such as theparameters measured by the one or more sensors of IMD 14 or informationrelated to therapies delivered to patient 12. For example, informationrelating to monitored physiological parameters of patient 12 can bestored in a memory of IMD 14 and periodically transmitted to externaldevice 16. Information can also be transmitted in the opposite direction(i.e. from the external device 16 to IMD 14), for example, when externaldevice 16 provides programming information to IMD 14.

External device 16 may process the information from IMD 14 to monitor acondition of patient 12. In the case of an implantable deviceincorporating a pressure sensor, for example, external device 16 mayreceive pressure measurements from IMD 14 and process pressuremeasurements to monitor for a cardiac condition, such as heart failure.As another example, external device 16 may process sensed cardiacsignals to monitor for a cardiac condition, such as tachycardia orbradycardia.

External device 16 may present the information to patient 12 via adisplay or other user interface. External device 16 may also relay theinformation received from IMD 14 to another IMD using intra-bodycommunication or other type of communication, e.g., inductive, magneticor radio frequency (RF) communication. Likewise, external device 16 mayrelay the information received from IMD 14 to an external device viaanother wireless communication scheme, such as RF communication,Bluetooth or the like. External device 16 may also transmit informationto IMD 14, such as information identifying a condition of patient 12,information sensed by a sensor of external device 16 or informationsensed by a sensor of another IMD implanted within patient 12. Theinformation transmitted to IMD 14 may, in some instances, controldelivery of therapy by IMD 14.

External device 16 may be a body worn device, such as a watch, necklace,armband, belt, ring, bracelet, patch, or other device that is configuredto be attached to, worn by, placed on or otherwise coupled to a body ofpatient 12 in order to contact electrodes 18 c and 18 d to the skin ofpatient 12. Alternatively, external device 16 may be a handheldcomputing device, such as a cellular telephone, smart phone, pager, orpersonal digital assistant (PDA), that includes electrodes 18 c and 18 dconfigured to be placed in contact with the skin of patient 12.

Although FIG. 1A is described in the context of a medical system 10having an IMD 14 communicating with an external device 16, IMD 14 mayalso communicate with another implantable medical device usingintra-body communication in a similar manner to that described above.

FIG. 1B is another conceptual diagram illustrating an example medicalsystem 10′ that includes another IMD 2 and another external device 4 inaddition to IMD 14 and external device 16. IMD 14 is illustrated in FIG.1B as being implanted within a pulmonary artery of the heart of patient12. Placement elsewhere throughout the vasculature of patient 12 is alsocontemplated by this disclosure. IMD 2 may, for example, be animplantable pacemaker, implantable cardioverter defibrillator (ICD),cardiac resynchronization therapy defibrillator (CRT-D), neurostimulatoror a combination thereof.

IMD 2 may be connected to one or more leads, such as leads 6 a and 6 b(collectively “leads 6”) implanted within a heart of patient 12 toprovide electrical stimulation therapy to the heart. Lead 6 a extendsfrom IMD 2 into a right atrium of patient 12 and lead 6 b extends fromthe IMD into the right ventricle of patient 12. Leads 6 each include oneor more electrodes. In the example illustrated in FIG. 1B, leads 6 eachinclude a respective tip electrode 7 a,b (collectively “tip electrodes7”), ring electrode 8 a,b (collectively “ring electrodes 8”), anddefibrillation electrode 9 a,b (collectively “defibrillation electrodes9”) located toward a distal end of their respective leads 6. Whenimplanted, tip electrodes 7, ring electrodes 8 and defibrillationelectrodes 9 are placed relative to or in a selected tissue, muscle,nerve or other location within patient 12. Leads 6 may include more orfewer electrodes than shown in FIG. 1B. As one example, one or both ofleads 6 may include a plurality of defibrillation electrodes, such as aright ventricular (RV) defibrillation electrode and a superior vena cava(SVC) defibrillation electrode. In another example, one or both of theleads may include multiple ring electrodes as is commonly used inmultipolar left ventricular leads. The configuration of electrodes,e.g., location, size, shape or the like, may vary based on the targetimplant location, type of disorder being treated, or the like.

Leads 6 are connected at a proximal end to IMD 2. IMD 2 illustrated inFIG. 1B includes a housing 11 within which electrical components and apower source of IMD 2 are housed. Housing 11 can be formed fromconductive materials, non-conductive materials or a combination thereofIMD 2 and/or housing 11 may include a connector block 13 configured tocouple to leads 6. Connector block 13 may include one or morereceptacles that interconnect with one or more connector terminalslocated on the proximal end of leads 6. Leads 6 are ultimatelyelectrically connected to one or more of the electrical componentswithin housing 11. One or more conductors (not shown in FIG. 1B) extendwithin each of leads 6 from connector block 13 along the length of thelead to engage the respective tip electrode 7, ring electrode 8 anddefibrillation electrode 9 of leads 6. In this manner, each of tipelectrodes 7, ring electrodes 8 and defibrillation electrodes 9 iselectrically coupled to a respective conductor within its associatedlead bodies. The respective conductors may electrically couple toelectrical circuitry, such as a therapy module or a sensing module, ofIMD 2 via connections in connector block 13.

In other examples, IMD 2 may be connected to more or fewer leadsextending from IMD 2. For example, IMD 2 may be coupled to three leads,e.g., a third lead implanted within a left ventricle of the heart ofpatient 12. In another example, IMD 2 may be coupled to a single leadthat is implanted within either an atrium or ventricle of the heart ofpatient 12. As such, IMD 2 may be used for single chamber ormulti-chamber cardiac rhythm management therapy. In further examples,implantable medical system 10 may include leads that are not implantedwithin the heart, but instead are implanted subcutaneously. In stillfurther examples, the IMD 2 may include no leads, but instead beimplanted within a chamber of the heart to provide leadless pacing.

As indicated above, housing 11 encloses a power source and electricalcomponents of IMD 2, such as one or more processors, memories,transmitters, receivers, transceivers, sensors, sensing circuitry,charging circuitry, therapy circuitry, antennas, and/or othercomponents. In the case of an implantable cardiac system, IMD 2 mayreceive electrical signals corresponding to electrical activity of theheart sensed using different electrode configurations and may processthe electrical signals to identify an arrhythmia of the heart. Inresponse to detecting an arrhythmia, IMD 2 selects a programmed therapyto treat the arrhythmia, e.g., pacing and/or defibrillation, anddelivers the therapy to the heart via the electrical conductors and oneor more of electrodes 7, 8 or 9. In the case of defibrillation therapy,for example, IMD 2 may deliver defibrillation shocks via defibrillationelectrodes 9.

IMD 2 may communicate with one or more other devices, including IMD 10,external device 6, and/or external device 4 to exchange data with theother devices. External device 4 may communicate with IMD 2 to configureIMD 2 to operate within a particular operating mode. For example,communications received from external device 4 may include one or moreoperating parameters for operation of IMD 2. IMD 2 may also transmitsensed physiological data, diagnostic determinations made based on thesensed physiological data, IMD performance data and/or IMD integritydata to external device 4. IMD 2 and external device 4 may communicatevia wireless communication using any techniques known in the art,including inductive telemetry or RF telemetry. However, othercommunication techniques are also contemplated.

IMD 14 and IMD 2 may communicate using intra-body communication. IMD 14may utilize a pair of electrodes in accordance with any of the examplesset forth herein as a communication dipole to transmit and, in someinstances, receive intra-body communications. IMD 2 may utilize alsoutilize a pair of electrodes as a communication dipole for receivingand, in some instances, transmitting intra-body communications. The pairof electrodes utilized by IMD 2 to transmit and receive communicationsmay be the same electrodes used to provide electrical stimulationtherapy, e.g., a pair of electrodes selected from electrodes 7, 8, 9,and/or a housing electrode. In this case, electrodes 7, 8, 9 and thehousing electrode may be electrically connected to an intra-bodycommunication module, e.g., by way of one or more switching modules. Inother examples, IMD 2 may have separate electrodes dedicated for use asa communication dipole for intra-body communication.

In one example, IMD 14 may transmit parameters measured by the one ormore sensors of IMD 14 to IMD 2. For example, information relating tomonitored physiological parameters of patient 12, such as the measuredpressure within the pulmonary artery, can be periodically transmittedfrom IMD 14 to IMD 2. IMD 2 may analyze the monitored physiologicalparameters measured by IMD 14 to identify any existing heart conditions,such as heart failure, and, if desirable, provide therapy and/or notifypatient 12 or physician of the condition. IMD 2 may analyze themonitored physiological parameters from IMD 14 independently or inconjunction with the electrical signals measured via electrodes 7, 8, 9of leads 6. IMD 2 may also relay the information received from IMD 14 toan external device, such as either of external device 6 and externaldevice 4. IMD 2 may relay the information via another communicationscheme, such as inductive telemetry, RF telemetry, Bluetooth or otherscheme. Alternatively, IMD 2 may relay the information to externaldevice 16 using intra-body communication.

As described above with respect to FIG. 1A, the radiation pattern of theintra-body communication emitted from IMD 10 is a generally toroid shapeabout the axis of the dipole, e.g., which may be defined by an imaginaryline extending between the effective location of electrodes of IMD 14.As such, the directionality of the radiation pattern is highly dependenton the orientation of the communication dipole. Unlike electrodes 18 cand 18 d of external device 16, the location of electrodes 7, 8, and 9of leads 6 may not be arranged in a particular orientation to provideoptimal intra-body communication. Instead, the location of electrodes 7,8 and 9 are selected to provide optimal or at least adequate electricalstimulation therapy to the heart. As such, the communication dipole ofIMD 14 may be designed in accordance with some aspects of thisdisclosure to allow for at least some control of directionality of theradiation pattern relative to electrodes of leads 6 of IMD 2.

As will be described in further detail herein, the location of theportion of fixation mechanism used as part of the dipole may be selectedsuch that the orientation of the axis of the communication dipole isoffset from the central axis of the vasculature when the IMD ispositioned within the vasculature. The term “offset” as used hereinrefers to the notion of the axis of the communication dipole is notsubstantially parallel with the central axis of the vasculature when theIMD is positioned within the vasculature. Instead, the axis of thecommunication dipole and the central axis of the vasculature when theIMD is positioned within the vasculature are offset at an angle relativeto one another at their point of intersection. Offsetting the axis ofthe communication dipole relative to the central axis of the vasculaturewhen the IMD is positioned within the vasculature provides thecapability to have more control of the orientation of the communicationdipole. For example, IMD 14 may be rotated during implantation tocontrol the orientation of the communication dipole and thus thedirectionality of the radiation pattern emitted by the communicationdipole of IMD 14. In instances in which housing of IMD 14 has alongitudinal axis that is substantially parallel to the vasculature whenthe IMD is positioned within the vasculature, the location of theportion of fixation mechanism used as part of the dipole may be selectedsuch that the axis of the communication dipole is offset from alongitudinal axis of the housing of IMD 14. In this manner, IMD 14 maybe oriented during implantation such that the directionality ofintra-body communication signals transmitted by the communication dipoleof IMD 14 are most likely to provide adequate coupling to thecommunication dipole of IMD 2 formed by electrodes of leads 6 ordedicated communication electrodes.

Although IMD 2 is described in the context of cardiac implantabledevices, the techniques of this disclosure are not so limited. Forexample, IMD 2 may alternatively be a non-cardiac implantable device,such as an implantable neurostimulator, drug pump, or other device thatprovides electrical stimulation therapy, drug therapy or any othertherapy to patient 12.

FIGS. 2 and 3A-3D are schematic diagrams illustrating an example IMD 20.IMD 20 may correspond with IMD 14 of FIGS. 1A or 1B. FIG. 2 illustratesIMD 20 implanted in a heart 21 of a patient 12. In the exampleillustrated in FIG. 2, IMD 20 is implanted in the pulmonary artery ofheart 21. However, IMD 20 may be placed within or near other portions ofheart 21, such as in one of the chambers (atrial or ventricular), veins,vessels, arteries or other vasculature of heart 21, such as the aorta,renal arteries, or inferior or superior vena cava. FIGS. 3A-3Dillustrate enlarged views of IMD 20 from various viewpoints. Inparticular, FIG. 3A illustrates an angled view from an aerialperspective, FIG. 3B illustrates a side view and FIGS. 3C and 3Dillustrate end views.

IMD 20 includes a housing 22 and a fixation mechanism 24. Housing 22 andfixation mechanism 24 of IMD 20 may be sized and shaped to fit within atarget location. In the example illustrated in FIGS. 2 and 3A-3D,housing 22 has a long, thin cylindrical shape (e.g., capsule-like shape)with rounded ends and a cylindrical sidewall extending between the endsto accommodate placement in the pulmonary artery of heart 21. Thisshape, for example, is considered to present low resistance to bloodflow. Other housing configurations may be employed, however, toaccommodate placement within or near other portions of heart 21 or otherlocations within the body of patient 12, the size and shape of IMD 20may vary based on the desired implant location. Additionally, the sizeand shape of housing 22 may vary depending on the number and type ofsensors incorporated within housing 22. For example, housing 22 may beformed in a different shape to accommodate placement within a chamber ofheart 21, along a spine, in a brain, or other location within or onpatient 12. As such, the techniques described in this disclosure shouldnot be limited by the shape of housing 22 described herein.

Housing 22 hermetically encloses components of IMD 20. Housing 22 may beformed in two sections 25 and 27 that can be hermetically sealed toprotect the components of IMD 20. Section 25 may contain a battery forpowering the electronics and section 27 may contain the electronics,e.g., at least one processor, memory, power source, communicationcircuitry, sensing circuitry, therapy circuitry or the like. For ease ofillustration, however, FIG. 3B illustrates only a communication module42 within section 27 of housing 22. However, other components of IMD 20,such as those described with respect to FIG. 9, may also be enclosedwithin section 27 of housing 22.

Housing 22 (or each of the sections 25 and 27) may be formed of any of avariety of biocompatible materials including biocompatible conductivematerials and biocompatible non-conductive materials. Examples ofbiocompatible, conductive materials include titanium (e.g., unalloyedtitanium with an American Society for Testing and Materials (ASTM) grade1 to grade 4 or an alloyed titanium (grade 5) that includes aluminum andvanadium), stainless steel, superalloy (such as a nonmagnetic,nickel-cobalt-chromium-molybdenum alloy), platinum or the like. Examplesof biocompatible, non-conductive materials include silicone,poly(p-xylylene) polymer sold under the trademark PARYLENE,polyurethane, epoxy, acetyl co-polymer plastics, PolyEtherEtherKetone(PEEK), liquid crystal polymer (LCP) plastics, ceramic, or the like.Housing 22 as well as some portions of fixation member 24 may beencapsulated in a biologically inert material such as a film of siliconeor poly(p-xylylene) polymer sold under the trademark PARYLENE.

Housing 22 also includes a sensor for sensing one or more parameters ofpatient 12. In the example illustrated in FIGS. 2 and 3A-3D, housing 22includes a pressure sensor 26 that obtains pressure measurements of anenvironment surrounding housing 22. Thus, IMD 20 may be an activeleadless pressure sensor system designed to continuously monitor bloodpressure and transmit the pressure measurements to external device 16 oranother implanted device. However, IMD 20 may sense pressuremeasurements of other locations of heart 21 depending on the location ofimplantation.

In the example illustrated in FIGS. 2 and 3A-3B, housing 22 is formed tohave an opening that exposes pressure sensor 26 to the environment atthe target location. The opening of housing 22 is illustrated in FIGS. 2and 3A-3B as being located along a length of housing 22. However, inother embodiments, the opening of housing 22 may be located on eitherend of housing 22. In any case, pressure sensor 26 is exposed to thesurrounding environment to obtain pressure measurements of thesurrounding environment.

Pressure sensor 26 may include a deformable diaphragm that moves inresponse to changes in the pressure of the environment to which it isexposed. Accordingly, there is a direct relationship between themovement of the diaphragm and the change in pressure. The diaphragm ofpressure sensor 26 may be positioned adjacent to the opening of housing22 so that pressure from the surrounding environment will act upon thediaphragm through the opening of housing 22. It is understood that inaccordance with one or more embodiments, the diaphragm may be acomponent of a capacitor structure used in generating capacitivemeasurements indicative of the pressure of the surrounding environment.In other words, pressure exerted on the diaphragm causes a correspondingmovement of the diaphragm which in turn alters a measured capacitance.As such, the measured capacitance corresponds to the pressure from thesurrounding environment acting on the diaphragm. By way of example onlyand without limitation, pressure sensor 26 may comprise a pressuresensor constructed in a manner similar to that described in commonlyassigned U.S. Pat. No. 6,221,024, entitled “Implantable Pressure Sensorand Method of Fabrication,” U.S. patent application Ser. No. 12/512,869filed Jul. 30, 2009 and entitled “Implantable Pressure Sensor withMembrane Bridge,” and U.S. Pat. No. 7,591,185, entitled “Pressure SensorConfigurations for Implantable Medical Electrical Leads” the contents ofeach of which are hereby incorporated by reference for their descriptionof pressure sensors.

Although described above as a capacitive pressure sensor, pressuresensor 26 may be any sort of pressure sensing device, such as anelectromagnetic pressure sensor that measures displacement of thediaphragm by means of changes in inductance (reluctance), linearvariable differential transformer (LVDT), Hall Effect or eddy currents,a piezoelectric pressure sensor, optical pressure sensor, or any otherpressure sensor. Housing 22 may include other types of sensors insteadof or in addition to pressure sensor 26, such as pH sensor, oxygensensor, temperature sensor, electrode, or any other type of sensor.

Fixation mechanism 24 affixes IMD 20 to the target location, such as thewall of the pulmonary artery in the example illustrated in FIG. 2.Fixation mechanism 24 of FIGS. 2 and 3A-3D is a generally tubular orcylindrical stent-like structure that is configured to lodge against avessel wall when implanted. Fixation mechanism 24 is configured suchthat housing 22 of IMD 20 is substantially adjacent to the wall of thevasculature when implanted. In other embodiments, fixation mechanism 24is configured such that housing 22 of IMD 20 is not in contact with thewall of the vasculature when implanted. Instead, housing 22 of IMD 20may be substantially radially centered within vasculature when implantedor otherwise offset from the wall of the vasculature.

Fixation mechanism 24 includes a plurality of struts 38 a-h that arearranged to form fixation mechanism 24. In particular, struts 38 a-h arearranged to form the stent-like structure having a lumen 40. The numberof struts and arrangement of struts may vary depending upon the desiredlength and structural rigidity of fixation mechanism 24. For examplewhen the target implant site is relatively short, it would be desirablefor fixation mechanism 24 to have a smaller number of struts arranged toform a short fixation mechanism. The material from which struts 38 a-38h are made may be capable of being manipulated such that fixationmechanism 24 may be radially compressed or otherwise manipulated to aidin delivery of IMD 20 to the target location. When located at the targetlocation, fixation mechanism may be expanded in situ, e.g., viainflation of a balloon (not shown), such that at least a portion ofstruts 38 securely engage the vessel wall. Struts 38 a-h may, forexample, be made from a variety conductive materials suitable forimplantation, including, but not limited to, nickel-titanium (nitinol),stainless steel, tantalum, nickel, titanium,nickel-cobalt-chromium-molybdenum “superalloy,” combinations of theabove, and the like.

In some embodiments, at least a portion of housing 22 of IMD 20 ispositioned within lumen 40 defined by fixation mechanism 24. Thediameter of lumen 40 is greater than the diameter of housing 22 suchthat the portion of housing 22 may be positioned within lumen 40 whilestill allowing blood to flow within the pulmonary artery. In the exampleillustrated in FIGS. 3A-3D, housing 22 of IMD 20 is completely locatedwithin lumen 40 defined by fixation mechanism 24. In other embodiments,only a portion of housing 22 may be located within lumen 40. Forexample, a portion of housing 22 forming first electrode 18 a may beextend beyond lumen 40. Disposing at least a portion of housing 22within lumen 40 reduces the overall length of IMD 20, which may beparticularly advantageous when IMD 20 is implanted at a target sitehaving a relatively short landing zone within the vessel. In furtherembodiments, however, none of housing 22 of IMD 20 may be positionedwithin lumen 40 defined by fixation mechanism 24.

Fixation mechanism 24 is mechanically coupled to housing 22 via strut 38h. In the example illustrated in FIGS. 3A and 3B, strut 38 h is coupledto the battery section 25 of housing 22. Strut 38 h may be mechanicallycoupled via crimping, welding or other technique. As described above,battery section 25 of housing 22 and fixation mechanism 24 may beconstructed from conductive material. In such instances, the mechanicalconnection between housing 22 and fixation mechanism 24 also results inan electrical coupling of housing 22 and fixation mechanism 24. Section25 of housing 22 may also serve as a ground plane for the electricalcomponents of IMD 14. In other instances, fixation mechanism 24 may beelectrically coupled to communication module 42 by one or moreelectrical interconnects within housing 22. In one embodiment, theelectrical connection to communication module 42 is made when strut 38 his mechanically coupled to housing 22.

As indicated with respect to FIGS. 1A and 1B, IMD 20 transmits and/orreceives wireless signals via intra-body communication using electrodes18 a and 18 b. To transmit wireless signals via intra-bodycommunication, IMD 20 applies a modulated current signal betweenelectrodes 18 a and 18 b, which causes a current to propagate into theconductive parts of the body (e.g., ion medium of extra- andintra-cellular fluids). The current induced in the body by electrodes 18a and 18 b results in a potential difference between electrodes 18 c and18 d of external device 16 (FIGS. 1A AND 1B) which are in contact withthe body of patient 12. To receive wireless signals via intra-bodycommunication, electrodes 18 a and 18 b of IMD 20 detect a potentialdifference caused by the introduction of current by external device 16.

As described above, IMD 20 is typically a small size to fit within thevasculature of patient 12. Conventionally, electrodes 18 a and 18 b usedfor intra-body communication are placed at opposite ends of housing 22.In this case, the maximum distance between electrodes 18 a and 18 b islimited to the length of housing 22. It is desirable, however, toincrease the distance between electrodes 18 a and 18 b to increase thestrength of the communication signal transmitted via intra-bodycommunication. In accordance with the techniques of this disclosure, IMD20 is configured to utilize a portion of fixation mechanism 24 as one orboth of the electrodes, thereby increasing the distance (L) separatingthe electrodes (sometimes referred to as the dipole length). Inaccordance with one aspect of this disclosure, a portion of housing 22is configured as first electrode 18 a and a portion of fixationmechanism 24 is configured as second electrode 18 b.

Housing 22 and fixation member 24 may be constructed of a biocompatibleconductive material, biocompatible non-conductive material or acombination thereof. In one example, housing 22 and fixation member 24may be constructed of a biocompatible conductive material that ispartially covered by a biocompatible non-conductive material. Theportions of housing 22 or fixation member 24 that are intended to serveas poles for intra-body wireless communication (e. g., to transmit orreceive RF signals) may remain uncovered. First electrode 18 a of thecommunication dipole may be formed by an end of housing 22. For example,housing 22 may be formed of a conductive material coated with anon-conductive coating that covers all of housing 22 except the endportion of housing 22 forming electrode 18 a (and possibly the exposedportion of sensor 26). The end portion of housing 22 may be electricallyisolated from the rest of the conductive housing via a non-conductivespacer 29. Electrode 18 a is electrically connected to communicationmodule 42 enclosed within housing 22 via an electrical interconnect,including, but not limited to a wire or conductive trace that extendsthrough the non-conductive spacer 29. In another example, housing 22 maybe constructed of a biocompatible, non-conductive material, such asborosilicate or other glass varieties, silicone or doped silicone,sapphire or ceramic (e.g., low temperature ceramic cofire (CLCC)materials) or a combination or conductive and non-conductive materialswith an electrode 18 a formed from a conductive material.

In the example illustrated in FIGS. 2 and 3A-3C, a portion of fixationmechanism 24 functions as second electrode 18 b of the communicationdipole. As described above, struts 38 a-h of fixation mechanism 24 maybe formed from an electrically conductive material 30 at least partiallycoated with a non-conductive dielectric material 28. In accordance withthe techniques of this disclosure, dielectric material 28 may beselectively applied such that only a portion of conductive material 30of fixation mechanism 24 is exposed to the surrounding environment. Therest of the conductive material 30 of fixation mechanism 24 is coveredby the dielectric material 28. Dielectric material 28 may includesilicone, parylene, polyurethane, epoxy, acetyl co-polymer plastics,PolyEtherEtherKetone (PEEK), liquid crystal polymer (LCP) plastics, orthe like, or a combination of dielectric materials. The thickness ofdielectric material 28 may depend on a number of factors, including theproperties of the dielectric material and the current amperage used forcommunication. In one example, the coating of dielectric material ofparylene may have a thickness of between approximately 2-20 microns.Again, however, the thickness of dielectric material 28 may vary andthis is just one example.

The exposed portion of fixation mechanism 24 (i.e., the electricallyconductive material 30 of fixation mechanism 24 not coated by dielectricmaterial 28) therefore functions as the second electrode 18 b forintra-body communication. Conductive material 30 of fixation mechanism24 and, more particularly strut 38 h of fixation mechanism 24, ismechanically and electrically coupled to housing 22, which serves as aground plane. As such, the exposed portion conductive material 30 offixation mechanism 24 is also at ground potential resulting in electrode18 b serving as a return path for the current injected into the body ofpatient 12 by the communication module 42 via electrode 18 a.

In this manner, the only portion of the conductive fixation mechanism 24that is exposed directly to the bodily fluid or tissue of patient 12 isthe portion of fixation mechanism 24 that functions as the secondelectrode 18 b. In the example illustrated in FIGS. 3A and 3B, a portionof electrically conductive material 30 of strut 38 g is exposed to thesurrounding environment while the remainder of the conductive material30 strut 38 g and the other struts 38 are covered by dielectric material28. The portion of the conductive material 30 of strut 38 that isexposed (i.e., not covered by dielectric material 28) is represented byshading. A portion of conductive material 30 of strut 38 h may also notbe covered by dielectric material 28 such that a good mechanical andelectrical connection may be made by way of the crimping or welding withhousing 22. However, that portion of conductive material 30 of strut 38h is located within housing 22 and therefore will not function as areturn path for the intra-body current injected by electrode 18 a.

By using a portion of fixation mechanism 24 as second electrode 18 b,the portion of fixation mechanism 24 forming second electrode 18 b is afurther distance from the first electrode than any other portion ofhousing 22, thus increasing the distance between electrodes 18 a and 18b and the effective dipole length. In some instances, the portion offixation mechanism 24 forming second electrode 18 b is located at aposition along fixation mechanism 24 that is the furthest distance fromthe portion of housing 22 forming first electrode 18 a, thus maximizingthe distance between electrodes 18 a and 18 b and the effective dipolelength.

Additionally, the location of the portion of fixation mechanism 24forming second electrode 18 b may be selected to provide at least somecontrol of the orientation of the communication dipole. In this manner,IMD 14 may be implanted with a more desirable orientation relative to acommunication dipole of another device, e.g., electrodes of one of leads6, which increases the robustness of communication between the devices.In instances in which electrodes 18 a and 18 b used for intra-bodycommunication are placed at opposite ends of housing 22 there is verylittle control over the orientation of the transit dipole of IMD 20.This is because the axis of the communication dipole of the housingelectrodes is substantially parallel with a central axis of thevasculature when the IMD is positioned within the vasculature. In theexample illustrated in FIGS. 3A-3D, the longitudinal axis defined byhousing 22 from one rounded end of housing 22 to the other rounded endof housing 22 is substantially parallel with the central axis of thevasculature of the pulmonary artery when the IMD is positioned withinthe vasculature. Thus, even if IMD 20 was rotated within the vasculatureof the heart during implantation, the orientation of electrodes 18 a and18 b relative to another point within or on the body (e.g., at which thecommunication dipole of the other device is located) remainssubstantially unchanged. However, selecting the portion of fixationmechanism 24 forming second electrode 18 b such that it is offset fromthe central axis of the vasculature when the IMD is positioned withinthe vasculature provides the capability to have more control of theorientation of the dipole (e.g., by rotating IMD 20 within thevasculature during implantation) relative to the other location withinor on the body of the other communication dipole.

In the example described with reference to FIGS. 3C and 3D, thelongitudinal axis of housing 22 is substantially parallel with thecentral axis of the vasculature when the IMD is positioned within thevasculature. The portion of fixation mechanism 24 forming secondelectrode 18 b is selected such that an axis of the dipole (labeled 42in FIG. 3C and 42′ in FIG. 3D) is offset relative to the longitudinalaxis of the housing 22 of IMD 20. As such, when IMD 20 is rotated withinthe vasculature of the patient, the orientation of the dipole and thedirectionality of the radiation pattern of the dipole may be adjusted.FIG. 3C may be viewed as a first position of IMD 20 in which theradiation pattern of the dipole is a generally toroid shape about axis42. FIG. 3D is a second position of IMD 20 within the vasculature, e.g.,rotated between approximately 45 degrees and 90 degrees within the samelocation of the vasculature as the first position. Rotating IMD 20 tothe second position within the vasculature rotates the axis of thedipole and thus the orientation of the dipole relative to a position inthe body. As such, a physician may adjust the orientation of IMD 20 toprovide better communication with a receive dipole of another device,e.g., by selecting an orientation that provides a desired directionalitythat has sufficient signal strength for transmitting to electrodes ofone of leads 6 of IMD 2. In one example, the portion of fixationmechanism 24 configured to function as electrode 18 b is selected suchthat there is a maximum offset relative to the central axis of thevasculature when the IMD is positioned within the vasculature.

In some instances, selecting a location of the portion of fixationmechanism 24 forming second electrode 18 b that provides at least somecontrol of the orientation of the transmitting dipole may result in areduction of the dipole length. As such, selection of the appropriateposition along fixation mechanism 24 to use as part of electrode 18 bmay be a tradeoff between dipole length and orientation controllability.In some instances, the position along fixation mechanism 24 to use aspart of electrode 18 b may be selected to maximize both the offset anddipole length.

In addition to the distance between electrodes 18 a and 18 b and theaxis of the dipole, the amount of conductive material 30 of fixationmechanism 24 that is exposed (i.e., not covered by the dielectricmaterial 28) also affects the effective dipole length. Additionally, theamount of conductive material 30 of fixation mechanism 24 that isexposed further affects the impedance of the dipole. In some instances,the amount of conductive material 30 of fixation mechanism 24 that isexposed may be increased beyond and optimal impedance to account forsome overgrowth effects, which would drive back the impedance to adesired range without having a significant effect on effective dipolelength. Such effects will be described in further detail with respect toFIGS. 6A-6C.

FIG. 3E illustrates an angled view of an IMD 20′ from an aerialperspective. IMD 20′ conforms substantially to IMD 20 of FIGS. 3A-3Dexcept that the portion of fixation mechanism 24 that functions assecond electrode 18 b is not exposed to the surrounding environment ofthe body. Instead, the portion of fixation mechanism 24 that functionsas second electrode 18 b of the communication dipole is also covered bya coating 31. Coating 31 may, for example, be deposited on the portionof fixation mechanism 24 that functions as second electrode 18 b usingany of a number of techniques known in the art, including plating,chemical or physical deposition.

Coating 31 may be designed to provide a capacitance associated with theportion of fixation mechanism 24 that functions as second electrode 18 bthat is significantly larger than a capacitance associated with the restof fixation mechanism 24. In this manner, the capacitance associatedwith the portion of fixation mechanism 24 that functions as secondelectrode 18 b dominates so that the effective impedance at thecommunication signal frequency does not limit the transmit signalamplitude. The desired minimum capacitance depends on the communicationfrequency, transmit amplitude, and output range of the transmitter. Forexample, with a 3 volt transmitter output range and a 1 mA transmitamplitude using communications in the approximately 100 kilohertzfrequency, a minimum capacitance of around 1 nanofarad would bedesirable.

In one aspect of this disclosure, coating 31 may be a dielectriccoating. For example, coating 31 may be the same material that coversthe other portions of fixation mechanism 24. In other words, coating 31may be the same dielectric material as dielectric material 28. In thiscase, the two materials have the same dielectric constant requiring athickness of coating 31 will be substantially thinner than a thicknessof dielectric material 28 to provide an increased capacitance. Thethickness of coating 31 will depend partially on the ratio of the amountof fixation mechanism coated with the thicker dielectric material 28 tothe amount of fixation mechanism coated with the thinner coating 31. Ifthis ratio is about 10:1, for example, and it is desirable to have thecapacitance of the portion of fixation mechanism 24 covered with coating31 to dominate by about an order of magnitude, a thickness ratio ofabout 100:1 may be desirable. In other words, dielectric material 28 maybe about 100 times thicker than coating 31. As such, the ratio of theamount of fixation mechanism covered by dielectric material 28 to theamount of fixation mechanism covered by coating 31 as well as thethickness of coating 31 or dielectric material 28 may be adjusted toachieve the desirable capacitance values.

In other instances, coating 31 may be a dielectric material having adifferent dielectric constant than dielectric material 28. In this case,the thicknesses of the coating 31 and dielectric material 28 may also bedependent on the dielectric constant of the materials used for coatings.For example, a dielectric material having a higher dielectric constantmay be selected for use as coating 31 to increase the capacitance of theportion of fixation mechanism 24 that functions as second electrode 18b. Using a dielectric having an increased dielectric constant may allowfor an increased thickness of the coating 31 covering the portion offixation mechanism 24 that functions as second electrode 18 b whilestill providing the ability to achieve the desirable capacitance ratios.

In one example, for an electrode 18 b having a 0.5 inch length of a 15mil fixation wire, a coating 31 having a thickness of 0.05 micrometersand dielectric constant of 3.4 may provide a capacitance ofapproximately 9.15 nanofarads at a frequency of approximately 100kilohertz and an impedance magnitude of approximately 175 Ohms.

In other aspects, coating 31 may a fractal coating or other method thatsubstantially increase surface area for a small section of the wire canmake capacitance substantially larger than the capacitance associatedwith the rest of fixation mechanism 24. Coating 31 may, for example, bea coating made up of one or more of a titanium nitride (TiNi), aplatinum oxide (e.g., PtO or Pt-black), iridium oxide (IrO), carbonnanotube, or other material. Fractal coatings that have been used onpacing and defibrillation electrodes may also be good candidates for useas coating 31 of fixation mechanism 24.

Although coating 31 is illustrated in FIG. 3E as coating only theportion of fixation mechanism 24 that is configured to function aselectrode 18 b, a similar coating (dielectric or fractal) may be appliedover electrode 18 a formed by the end of housing 22. Additionally, theportion of fixation mechanism 24 configured to function as part ofelectrode 18 b may be selected such that the orientation of the axis ofthe communication dipole may be offset from the central axis of thevasculature when the IMD is positioned within the vasculature to providemore control of the orientation of the dipole as described in moredetail with respect to FIGS. 3C and 3D.

FIG. 3F illustrates an angled view of an IMD 20″ from an aerialperspective. IMD 20″ conforms substantially with IMD 20 of FIGS. 3A-3Dexcept that a second portion of housing 22 functions in conjunction withthe exposed portion of fixation mechanism 24 as second electrode 18 b.In the example illustrated in FIG. 3F, the second end of housing 22,opposite the end that functions as electrode 18 a, functions as part ofsecond electrode 18 b.

In this case, the coating of non-conductive material covering housing 22may be applied or removed such that the other end of housing 22 isexposed. In one example, an overlay (e.g., of silicon or otherinsulator) may be placed over the sensor with backfilled adhesive. Theoverlay may be designed to make use of the entire length of the sensorfor creating a further separation between the poles. In other words, amajor portion of section 27 (which may be conductive) is covered leavingonly the distal end to function as part of electrode 18 b. In oneinstance, the overlay may be shaped for the features around the sensorso that excessive backfill is not required. In other examples, thecoating covering housing 22 may be applied using other coatingtechniques including chemical vapor deposition, physical vapordeposition, chemical and electrochemical techniques, dip coating, or anyother coating technique known in the field of coating.

This configuration may have several advantages. Forming second electrode18 b using a portion of housing 22 and a portion of fixation mechanism24 may provide an increased reliability of communication dipole. Forexample, tissue growth over the exposed portion of fixation mechanism 24may affect the impedance and transmission efficiency of thecommunication dipole. Additionally, oxidation or corrosion over theconnection between housing 22 and fixation mechanism 24 may affect theelectrical connection with fixation structure 24. As another example,such a configuration may also provide the capability to provide somecontrol over orientation of the communication dipole (e.g., duringimplant), as described in detail above.

These advantages, however, come with a reduction of the effective dipolelength.

The effective dipole length of the communication dipole may generally bedefined as

$\frac{{{D\; 1*A\; 1} + {D\; 2*A\; 2}},}{{A\; 1} + {A\; 2}}$wherein D1 is the distance from electrode 18 a to the second exposedportion of housing 22, A1 is the area of the second exposed portion ofhousing 22, D2 is the distance from electrode 18 a to the exposedportion of fixation mechanism 24, and A2 is the area of the exposedportion of fixation mechanism 24. The sizes of the second exposedportion of housing 22 and the exposed portion of fixation mechanism 24may be selected to maximize the effective impedance. D1 and D2 may, insome instance, be vector values.

In some instances, a coating similar to coating 31 described withrespect to FIG. 3E may cover the portion of fixation mechanism 24 thatis configured to function as part of electrode 18 b, the portion ofhousing 22 configured to function as electrode 18 b, and/or the portionof housing 22 configured to function as electrode 18 a. Additionally,the portion of fixation mechanism 24 configured to function as part ofelectrode 18 a may be selected such that the orientation of the axis ofthe communication dipole may be offset relative to the central axis ofthe vasculature when the IMD is positioned within the vasculature toprovide more control of the orientation of the dipole as described inmore detail with respect to FIGS. 3C and 3D.

Although this disclosure is described with respect to IMD 20 being animplantable pressure sensor implanted within a heart of patient 12, IMD20 be placed in locations within patient 12, such as within or proximateto a spinal cord, brain, stomach, or pelvic floor, and may sense,sample, and process any of a variety of parameters such as heartactivity, muscle activity, brain electrical activity, intravascularpressure, blood pressure, blood flow, acceleration, displacement,motion, respiration, or blood/tissue chemistry, such as oxygensaturation, carbon dioxide, pH, protein levels, enzyme levels or otherparameter or combination of parameters. IMD 20 transmits the sensedparameters to another device, such as external device 16 (FIGS. 1A AND1B) or another IMD 2 (FIG. 1B), which may in turn monitor a condition ofpatient 12 or provide therapy to patient 12 as a function of the sensedparameters.

Although illustrated as a stent-like fixation mechanism in FIGS. 2 and3A-3B, fixation mechanism 24 may be a different fixation mechanism thatexerts enough force against, embeds within, extends through or otherwiseaffixes IMD 20 to the target location. Other fixation mechanisms mayinclude one or more tines, loops, or other mechanism that may be used toaffix IMD 20 to the target location, some of which are illustrated anddescribed in FIGS. 4, 5, 7 and 8.

FIG. 4A is a schematic diagram illustrating another example IMD 50. IMD50 is similar to IMD 20 of FIGS. 3A-3D, but includes a differentfixation element 54. Fixation element 54 is another stent-like fixationelement composed of a number of conductive struts. In the exampleillustrated in FIG. 4A, housing 22 of IMD 50 is not positioned withinthe lumen defined by fixation mechanism 54.

Like fixation mechanism 24, a portion of fixation mechanism 54 isconfigured as second electrode 18 b. In particular, fixation mechanism54 includes a dielectric material that covers a majority of fixationmechanism 54, but is selectively applied or removed such that a portionof fixation mechanism 54 is exposed to the surrounding environment tofunction as the second electrode 18 b for intra-body communication. Therest of fixation mechanism 54 is covered by the dielectric material,except possibly the portion of fixation mechanism that is mechanicallyand electrically coupled to housing 22. The portion of fixationmechanism 54 that is exposed is represented by the shaded portion offixation mechanism 54.

In some instances, a coating similar to coating 31 described withrespect to FIG. 3E may cover the portion of fixation mechanism 54 thatis configured to function as electrode 18 b and/or the portion ofhousing 22 configured to function as electrode 18 a. Additionally, theportion of fixation mechanism 54 configured to function as part ofelectrode 18 b may be selected such that the orientation of the axis ofthe communication dipole may be offset from the central axis of thevasculature when the IMD is positioned within the vasculature to providemore control of the orientation of the dipole as described in moredetail with respect to FIGS. 3C and 3D.

FIG. 4B is a schematic diagram illustrating another example IMD 50′. IMD50′ is similar to IMD 20″ of FIG. 3F, but includes a different fixationelement 54 similar the fixation element described above with respect toFIG. 4A. IMD 50′ includes a second portion of housing 22 that functionsin conjunction with the exposed portion of fixation mechanism 54 assecond electrode 18 b. In the example illustrated in FIG. 4B, the secondend of housing 22, opposite the end that functions as electrode 18 a,functions as part of second electrode 18 b. In this case, the coating ofnon-conductive material covering housing 22 may be applied or removedsuch that the other end of housing 22 is exposed.

In some instances, a coating similar to coating 31 described withrespect to FIG. 3E may cover the portion of fixation mechanism 54 thatis configured to function as part of electrode 18 b, the portion ofhousing 22 configured to function as electrode 18 b, and/or the portionof housing 22 configured to function as electrode 18 a. Additionally,the portion of fixation mechanism 54 configured to function as part ofelectrode 18 a may be selected such that the orientation of the axis ofthe communication dipole may be offset from the central axis of thevasculature when the IMD is positioned within the vasculature to providemore control of the orientation of the dipole as described in moredetail with respect to FIGS. 3C and 3D.

FIG. 5A and 5B are schematic diagrams illustrating another example IMD60. IMD 60 includes a housing 22 that is described above with respect toIMD 20 of FIGS. 3A-3C. However, IMD 60 includes a different fixationmechanism 64. Fixation mechanism 64 includes a pair of longitudinallyspaced loops 62 a and 62 b (collectively “loops 6”) connected by anelongate linear attachment strut 65. Loops 62 are spaced apartsufficiently to receive and, in some instances, embrace housing 22 withthe housing extending lengthwise between loops 62. First loop 62 aextends from a first end of housing 22 and loop 62 b extends from asecond, opposite end of housing 22. Loops 62 a and 62 b affix IMD 60within the vasculature due to force applied to the vessel wall by therespective loops 62 a and 62 b pushing radially against the vessel.Although illustrated in FIGS. 5A and 5B as including two loops 62 a and62 b, fixation mechanism 64 of IMD 60 may include only a single fixationloop (e.g., only loop 62 a) or more than two fixation loops.

Fixation member 64, including the attachment strut 65, may be formedfrom a sheet of conductive material by laser cutting or electrochemicaletching or other fabricating techniques known in the art. The resultingfixation member 64 is formed as a single, integral piece. In otherexamples, fixation member 64 may be formed of multiple pieces that aremechanically connected (e.g., via welding) to form an integral piece.The wire-like elements that make up the loops 62 and the attachmentstrut 65 may have a circular cross section or a non-circular crosssection (square or rectangular) and may have a substantially uniformthickness.

The conductive material of loops 62 and attachment strut 65 are at leastpartially covered by a dielectric material. In the example illustratedin FIGS. 5A and 5B all of fixation member 64 is covered by anon-conductive dielectric material except a portion of loop 62 b that isexposed (represented by the shaded portion of loop 62 b) and functionsas second electrode 18 b for intra-body communication in conjunctionwith electrode 18 a that is formed at the end of housing 22. The rest ofloop 62 b and the entire loop 62 a are covered by the dielectricmaterial. Attachment strut 65 is also covered by the dielectric materialexcept for a portion of attachment strut 65 that is mechanicallyconnected to housing 22. However, the portion of housing 22 thatattaches to attachment strut 65 may be covered by a non-conductivematerial so that the point of fixation does not function as a returnpath for the current injected into the body of patient 12 by thecommunication module 42 via electrode 18 a.

In some instances, a coating similar to coating 31 described withrespect to FIG. 3E may cover the portion of fixation mechanism 64 thatis configured to function as part of electrode 18 b and/or the portionof housing 22 configured to function as electrode 18 a. Additionally,the portion of fixation mechanism 64 configured to function as part ofelectrode 18 b may be selected such that the orientation of the axis ofthe communication dipole may be offset from the central axis of thevasculature when the IMD is positioned within the vasculature to providemore control of the orientation of the dipole as described in moredetail with respect to FIGS. 3C and 3D.

FIG. 5C is a schematic diagram illustrating another example IMD 60′. IMD60′ is similar to IMD 60 of FIGS. 5A and 5B except IMD 60′ includes asecond portion of housing 22 that functions in conjunction with theexposed portion of fixation mechanism 64 as second electrode 18 b. Inthe example illustrated in FIG. 5C, the second end of housing 22 that isopposite the end that functions as electrode 18 a is utilized as part ofsecond electrode 18 b. In this case, the coating of non-conductivematerial covering housing 22 may be applied or removed such that bothends of housing 22 are exposed.

In some instances, a coating similar to coating 31 described withrespect to FIG. 3E may cover the portion of fixation mechanism 64 thatis configured to function as part of electrode 18 b, the portion ofhousing 22 configured to function as electrode 18 b, and/or the portionof housing 22 configured to function as electrode 18 a. Additionally,the portion of fixation mechanism 64 configured to function as part ofelectrode 18 b may be selected such that the orientation of the axis ofthe communication dipole may be offset from the central axis of thevasculature when the IMD is positioned within the vasculature to providemore control of the orientation of the dipole as described in moredetail with respect to FIGS. 3C and 3D.

FIG. 6A is a graph illustrating an example plot of effective dipolelength and impedance versus the amount of fixation mechanism 64 that isexposed. The plots in the graph of FIG. 6A are mathematically simulatedresults for an IMD having a communication dipole similar to thatdescribed above with respect to IMD 60′ of FIG. 5C. In particular, oneend of housing 22 is configured to function as electrode 18 a and thecombination of a second end of housing 22 and the exposed portion offixation mechanism is configured to function as second electrode 18 b.

The x-axis of the graph in FIG. 6A corresponds with the length (mm) offixation mechanism that is exposed. The y-axis on the left hand side ofthe graph of FIG. 6A corresponds to effective dipole length (mm) and they-axis on the right hand side corresponds with combined electrodeimpedance in blood (ohms) The effective dipole length is plotted as asolid line and the combined electrode impedance is plotted as a dottedline.

As illustrated in FIG. 6A, the effective dipole length increasessomewhat exponentially as the length of the exposed fixation mechanism64 increases. The increase in effective dipole length continues untilthe point at which the transition from exposed to insulated fixation isat the same point as the effective dipole length. After this point,exposing additional fixation material beyond this point will decreasethe effective dipole length. As further illustrated in FIG. 6A, theimpedance decreases somewhat exponentially as the length of the exposedfixation mechanism 64 increases. The decrease in impedance continuesbeyond the point at which the transition from exposed to insulatedfixation is at the same point as the effective dipole length. The amountof fixation mechanism 64 that is exposed may be selected to obtain adesired balance between effective dipole length and combined electrodeimpedance.

The relationship between the effective dipole length vs. exposed lengthis strongly dependent is highly dependent on the shape of the fixationmechanism. Different shapes of fixation mechanisms may have effectivedipole length curves that vary considerably in shape. The exampleillustrated in FIG. 6A is just one example of a simulation of an IMDthat is similar to IMD 60′.

FIG. 6B is a graph illustrating an example plot of effective dipolelength and impedance versus the amount of fixation mechanism 64 that isexposed. The plots in the graph of FIG. 6B are mathematically simulatedresults for an IMD having a communication dipole similar to thatdescribed above with respect to IMD 60 of FIGS. 5A and 5B. Inparticular, one end of housing 22 is configured to function as electrode18 a and an exposed portion of fixation mechanism is configured tofunction as second electrode 18 b.

The x-axis of the graph in FIG. 6B corresponds with the length (mm) offixation mechanism that is exposed. The y-axis on the left hand side ofthe graph of FIG. 6B corresponds to effective dipole length (mm) and they-axis on the right hand side corresponds with electrode impedance inblood (ohms) The effective dipole length is plotted as a solid line andthe combined electrode impedance is plotted as a dotted line.

As illustrated in FIG. 6B, the effective dipole length decreases in asomewhat linear manner as the length of the exposed fixation mechanism64 increases. As further illustrated in FIG. 6B, the impedance decreasessomewhat exponentially as the length of the exposed fixation mechanism64 increases. The amount of fixation mechanism 64 that is exposed may beselected to obtain a desired balance between effective dipole length andcombined electrode impedance.

The relationship between the effective dipole length vs. exposed lengthis strongly dependent is highly dependent on the shape of the fixationmechanism. Different shapes of fixation mechanisms may have effectivedipole length curves that vary considerably in shape. The exampleillustrated in FIG. 6B is just one example of a simulation of an IMDthat is similar to IMD 60 of FIGS. 5A and 5B.

FIG. 6C is a graph illustrating an example plot of effective dipolelength and impedance versus the amount of fixation mechanism that isexposed for an example IMD. The x-axis of the graph in FIG. 6Ccorresponds with percentage of the linear distance of fixation mechanismthat is exposed. The y-axis on the left hand side of the graph of FIG.6C corresponds to effective dipole length (mm) and the y-axis on theright hand side corresponds with impedance in blood (Ohms in blood).

As illustrated in FIG. 6C, the effective dipole length (represented bythe dotted line) increases exponentially as the percentage of the lineardistance of fixation mechanism 24 that is exposed increases untilapproximately 20% (which may correspond with the point at which thetransition from exposed to insulated fixation is at the same point asthe effective dipole length) and then begins to linearly decrease as thepercentage of the linear distance of fixation mechanism 24 that isexposed continues to increase. As further illustrated in FIG. 6C, theimpedance (represented by the solid line) decreases exponentially as thepercentage of the linear distance of fixation mechanism 24 that isexposed increases. As such, the amount of fixation mechanism 24 that isexposed may be selected to obtain a balance between effective dipolelength and impedance. In one example, the percentage of fixationmechanism 24 that is exposed may be between approximately 5-30% and,more preferably between approximately 10-20%. As will be described withrespect to FIGS. 8 a-8 c, more than one electrode formed by a portion offixation mechanism 24 may be turned on to increase the linear distanceof fixation mechanism 24 and thereby affect the impedance.

FIG. 7 is a schematic diagram illustrating another example IMD 70. IMD70 is similar to IMD 60 of FIGS. 5A and 5B, but first electrode 18 a isformed by a portion of loop 62 a instead of by an end of housing 22. Inthis case, a portion of loop 62 a is not covered by the dielectricmaterial such that the exposed portion of loop 62 a (represented as theshaded portion of loop 62 a) functions as first electrode 18 a forintra-body communication. To enable operation of this configuration,loops 62 are electrically isolated from one another, e.g., either byinserting a non-conductive material along a portion of attachment strut65 or constructing loops 62 out of separate pieces of conductivematerial that are not electrically coupled to one another. The rest ofloop 62 a is covered by the dielectric material. Loop 62 a iselectrically connected to the communication module of IMD 70, e.g.,utilizing one or more electrical interconnects or feed throughs, totransmit and receive signals in conjunction with electrode 18 b formedby loop 62 b. Utilizing portions of fixation mechanism 64 as bothelectrodes 18 a and 18 b may result in an even larger distance betweenelectrodes 18 a and 18 b, thereby further extending the effective dipolelength. One or both of electrodes 18 a or 18 b may also utilize anexposed portion of housing 22 as described above with respect to FIG.3F, FIG. 4B, and FIG. 5C as further illustrated in FIG. 7B.

In further aspects, a coating similar to coating 31 described withrespect to FIG. 3E may cover the portion of fixation mechanism 64 thatis configured to function as part of electrode 18 a or the portion offixation mechanism 64 that is configured to function as part ofelectrode 18 b. In some instances, a coating similar to coating 31described with respect to FIG. 3E may cover the portion of fixationmechanism 64 that is configured to function as part of electrode 18 b,the portion of housing 22 configured to function as electrode 18 b,and/or the portion of housing 22 configured to function as electrode 18a. Additionally, the portion of fixation mechanism 64 configured tofunction as part of electrode 18 a or 18 b may be selected such that theorientation of the axis of the communication dipole may be offset fromthe central axis of the vasculature when the IMD is positioned withinthe vasculature to provide more control of the orientation of the dipoleas described in more detail with respect to FIGS. 3C and 3D.

FIG. 7B is a schematic diagram illustrating another example IMD 70′. IMD70′ is similar to IMD 70 of FIG. 7A, but first electrode 18 a is formedby a portion of loop 62 a instead of by an end of housing 22. IMD 70′ issimilar to IMD 70 of FIG. 7A except IMD 70′ includes a first exposedportion of housing 22 that functions in conjunction with the exposedportion of loop 62 a as first electrode 18 a and a second exposedportion of housing 22 that functions in conjunction with the exposedportion of loop 62 b as second electrode 18 b. In this case, loop 62 ais electrically and possibly mechanically coupled to the first exposedend of housing 22 and loop 62 b is electrically and possiblymechanically coupled to the second exposed end of housing 22.

In further aspects, a coating similar to coating 31 described withrespect to FIG. 3E may cover the portion of fixation mechanism 64 thatis configured to function as part of electrode 18 a, the portion offixation mechanism 64 that is configured to function as part ofelectrode 18 b, the portion of housing 22 configured to function aselectrode 18 a, and/or the portion of housing 22 configured to functionas electrode 18 b. Additionally, the portion of fixation mechanism 64configured to function as part of electrode 18 a or 18 b may be selectedsuch that the orientation of the axis of the communication dipole may beoffset from the central axis of the vasculature when the IMD ispositioned within the vasculature to provide more control of theorientation of the dipole as described in more detail with respect toFIGS. 3C and 3D.

FIGS. 8A-8C illustrate a further example of an IMD 80. IMD 80 includes ahousing 22 substantially similar to the housing described in detailabove with respect to FIGS. 3A-3C. IMD 80 also includes a fixationmechanism 84 that is similar to fixation mechanism 24 of IMD 20illustrated in FIGS. 3A-3C. Housing 22 of IMD 80 is positioned partiallywithin the lumen defined by fixation mechanism 84 instead of beingcompletely located within the lumen as illustrated in FIGS. 3A-3C.

In the example illustrated in FIGS. 8A-8C, the end of housing 22 doesnot function as an electrode used for intra-body communication. Instead,a plurality of struts 82 a-d extend from the end of housing 22. Struts82 a-d are formed of a conductive material that is partially covered bya dielectric material such that only a portion of struts 82 a-d areexposed to the surrounding environment to function as electrodes for usein intra-body communication. In the example illustrated in FIGS. 8A-8C,a distal end of each of struts 82 a-d is exposed to the surroundingenvironment to form electrodes 18 e-h, respectively. Struts 82 a-d mayor may not additionally function as part of the fixation mechanism forfixating IMD 80 to the target location.

As shown in the end view illustrated in FIG. 8C, each of struts 82 a-dis attached to housing 22 by a separate feed through. In this manner,each of the electrodes 18 e-h associated with respective struts 82 a-dis electrically isolated from one another. Struts 82 a-d areelectrically coupled to the communication module of IMD 80 such that anyof electrodes 18 e-h may be used for intra-body communication inconjunction with electrode 18 a formed by an exposed portion of fixationmechanism 24 or one another. Struts 82 a-d may be electrically coupledto the communication module (not shown) via a switching device (notshown) that may selectively couple one of the electrodes 18 e-h to thecommunication module of IMD 80. In this manner, IMD 80 has the abilityto switch electrode configurations used for intra-body communication,thereby providing transmit dipole and receive dipole diversity.

The signal received by external device 16 at electrodes 18 c and 18 d(FIGS. 1A AND 1B), which corresponds to the electric potentialdifference between electrodes 18 c and 18 d, is a function of the lengthof the transmitting dipole, the length of the receive dipole, and theangle of orientation between the transmit dipole and receive dipole. Theangle of orientation can be altered due to varying locations andorientations of IMD 80 or the different geometries of individualpatients. Thus, no one single transmit or receive dipole will be optimalfor all implant scenarios, particularly for enabling placement ofexternal device 16 in an ergonomical manner.

Selecting among the plurality of electrodes 18 e-h enables IMD 80 toadjust the angle of orientation between the dipole of IMD 80 and thedipole of external device 16. Even a slight adjustment of the angle oforientation, e.g., by switching from a dipole formed by electrodes 18 ato 18 e to a dipole formed by electrodes 18 a and 18 g may improve thequality and reliability of communication with external device 16.Moreover, such ability to adjust the angle of orientation between thedipole of IMD 80 and the dipole of external device 16 may allow for useof more ergonomical external devices, e.g., body-worn devices.

IMD 80 may selectively couple one of the electrodes 18 e-h to thecommunication module of IMD 80 to function as the transmit and receivedipole in conjunction with electrode 18 a. In one example, externaldevice 16 may assess the signal quality of a received signal and send acommand to IMD 80 to reconfigure the switch to couple to a different oneof electrodes 18 e-h when the signal quality is not sufficient. Inanother example, IMD 80 may assess the signal quality of a receivedsignal and reconfigure the switch based on the assessment. In thismanner, IMD 80 may be selectively configured between different dipolearrangements formed by electrodes positioned at different positions toprovide a desirable signal quality for communication with an implantablemedical device. The signal quality may be assessed using a variety ofmethods including but not limited to a transmission power required forsignal detection, a received signal strength, a received signal-to-noiseratio, a bit error rate, a data throughput rate, a data dropout rate, abackground noise floor, an optimum frequency, a correlation between adetected signal and a known template for a signal, or any combination ofthese measures.

It may be desirable to have a surface area of the electrodes used forintra-body communication be about the same size or a ratio of the largerelectrode. In some examples, IMD 80 may connect the communication moduleto more than two electrodes. For example, IMD 80 may connect thecommunication module to electrode 18 a and two of electrodes 18 e-hconcurrently (for a total of three electrodes) to change the effectiveelectrode surface area and thus the impedance. In other words, byelectrically connecting to three electrodes, the total surface area ofthe exposed fixation mechanism is increased thereby affecting theimpedance. In this manner, IMD 80 may selectively adjust the impedanceby selecting more or fewer electrodes. This may be particularlyadvantageous if the surface area of one of the electrodes forcommunication changes, e.g., due damage to the dielectric materialsomewhere along the fixation mechanism causing additional surface areato be exposed or tissue overgrowth that covers a portion of the exposedfixation mechanism decreasing the surface area of conductive fixationmechanism exposed.

In the example illustrated in FIG. 8, the length of the exposed portionof struts 82 a-d forming electrodes 18 e-h, respectively, areapproximately the same. However, in other instances, the length of theexposed portion of each of struts 82 a-d forming electrodes 18 e-h (or aportion thereof) may be of different lengths. IMD 80 may selectivelycouple one of the electrodes 18 e-h (formed by the exposed portion ofstruts 82 a-d, respectively) to the communication module of IMD 80 toachieve a desired impedance or dipole length. In this manner, IMD 80 mayselectively change the effective electrode surface area and thus theimpedance. IMD 80 may, for example, make such an adjustmentautomatically upon initiating communication, make the adjustment inresponse to a signal quality below a certain level, or make theadjustment in response to a command from an another device (external orimplanted).

Although illustrated in FIGS. 8A-8C as including four struts 82 a-dextending from the end of housing 22, IMD 80 may include more of fewerstruts 82. In fact, in one example IMD 80 may include only a singlestrut 82 that extends from the end of housing 22 (e.g., strut 82 d) toincrease the length of the dipole. In such a case, however, the IMD 20does not provide dipole diversity.

In further aspects, a coating similar to coating 31 described withrespect to FIG. 3E may cover electrodes 18 a-e and/or any portions ofhousing 22 configured to function as part of one of the electrodes.Additionally, the portion of struts 82 a-e configured to function aspart of electrodes 18 a-e may be selected such that the orientation ofthe axis of the communication dipole may be offset from the central axisof the vasculature when the IMD is positioned within the vasculature toprovide more control of the orientation of the dipole as described inmore detail with respect to FIGS. 3C and 3D.

FIG. 9 is a functional block diagram illustrating components of animplantable medical device in further detail. FIG. 9 will be describedwith respect to IMD 20 for purposes of illustration. However, theimplantable medical device may correspond to any of the otherimplantable medical devices described herein.

IMD 20 includes a pressure sensor 26, communication module 42,electrodes 18 a and 18 b, processor 44, memory 46 and power source 48.The components of IMD 20 are shown to be interconnected by adata/communication bus 49, but may be interconnected by other meansincluding direct electrical or non-electrical connections or acombination of different types of connections.

As described above, IMD 20 may sense one or more parameters (e.g.,physiological or biological parameters) of patient 12 and/or detect oneor more conditions from the sensed parameters. For example, pressuresensor 26 may be configured to obtain signals related to the pressure ofthe surrounding environment within which IMD 20 is implanted. Althoughdescribed with respect to IMD 20 including pressure sensor 26, IMD 20may include any number and type of sensors depending on the type ofdevice, including a pH sensor, oxygen sensor, temperature sensor,electrodes, or any other type of sensor.

The parameters sensed by pressure sensor 26 may be stored in memory 46.In some instances, the sensed parameters may be stored in raw form. Inother instances, the sensed parameters may be processed and theprocessed parameters may be stored in memory 46. For example, IMD 20 mayinclude one or more analog or digital components that amplify and filterthe sensed parameters and store the filtered parameters in memory 46.The parameters stored in memory 46 may, in some cases, be retrieved andfurther processed by processor 44. Processor 44 may, for example,process the sensed parameters to monitor or detect a condition ofpatient 12.

Processor 44 may control operation of IMD 20 with the aid ofinstructions associated with program information stored in memory 46.For example, the instructions may define the timing at which to samplesignal from pressure sensor 26 or, in instances in which implantablemedical device 20 delivery therapy, the timing of therapy delivery,waveform characteristics for electrical stimulation, and/or dosingprograms that specify an amount of a therapeutic agent to be deliveredto a target tissue site within patient 12. Processor 44 may also controloperation of communication module 42 to transmit communications toand/or receive communications from another medical device, such asexternal device 16 (FIGS. 1A AND 1B) or another implanted medicaldevice.

Communication module 42 is coupled to at least two electrodes 18 a and18 b configured to function as an electric dipole and transmit andreceive information encoded in electrical signals to and from externaldevice 16. The electrical signals are typically transmitted and receivedin a modulated format such as frequency shift keying, amplitude shiftkeying, phase shift keying, pulse width modulation, pulse amplitudemodulation, quadrature amplitude modulation, orthogonal frequencydivision multiplexing, spread spectrum techniques, or in an analogsignal format and/or modulation technique such as analog amplitudemodulation or frequency modulation. In some embodiments, thecommunication module 42 of IMD 14 can be configured to operate forperiods of time in a sleep state in order to conserve battery power. Insuch a configuration, communication module 42 may be configured to wakeup periodically to listen to a communication request from externaldevice 16 or to transmit the stored parameters sensed by pressure sensor26.

Communication module 42 may include any suitable hardware, firmware,software or any combination thereof for communicating with anotherdevice for transmitting and receiving intra-body communications. Forexample, communication module 42 may include a current source,modulator, demodulator, encoder, decoder, amplifier, frequencyconverter, filter or any other component desired for communicating usingintra-body communication techniques.

Power source 48 delivers operating power to various components of IMD20. Power source 48 may include, for example, a small rechargeable ornon-rechargeable battery and a power generation circuit to produce theoperating power. In some examples, power requirements may be smallenough to allow IMD 20 to utilize patient motion and implement a kineticenergy-scavenging device to trickle charge a rechargeable battery. Inother examples, traditional batteries may be used for a limited periodof time. As a further alternative, an external inductive power supplymay transcutaneously power IMD 20 whenever measurements are needed ordesired.

IMD 20 of FIG. 9 is provided for purposes of illustration. IMD 20 mayinclude more or fewer components than those illustrated in FIG. 9. Forexample, IMD 20 may include more than two electrodes coupled tocommunication module 42. Such an embodiment is described with respect toFIGS. 8A-8C. In such an embodiment, communication module 42 may beselectively configured to couple to the two electrodes of the pluralityof electrodes that provide an adequate orientation with respect to thedipole of external device 16. To this end, communication module 42 maybe coupled to the electrodes via a switching device (not shown) that maybe configured to couple to the selected electrodes. Processor 44 may,for example, control the configuration of the switching device inresponse to a command from external device 16. In another example,communication module 42 or processor 44 may be configured to operate asa signal quality monitor and assess the signal quality of an electricalsignal received from external device 16. In this case, communicationmodule 42 or processor 44 may control the configuration of the switchingdevice in response to the signal quality assessment to achieve dipolediversity.

As another example, IMD 20 may be an implantable medical deviceconfigured to also provide therapy, such as electrical stimulationtherapy or drug delivery therapy, in accordance with parameters of oneor more selected therapy programs. In this case, implantable sensor mayinclude a therapy module (not shown) to generate therapy according toone or more therapy programs. In the case of electrical stimulationtherapy, the therapy module may include a stimulation generator thatgenerates and delivers electrical stimulation therapy, e.g., in the formof pulses or shocks. Processor 44 may control the stimulation generatorto deliver electrical stimulation pulses with amplitudes, pulse widths,frequency, and/or electrode polarities specified by the one or moretherapy programs. In the case of drug delivery therapy, the therapymodule may include a pump that delivers a drug or therapeutic agent,e.g., via a catheter or other delivery mechanism. Processor 44 maycontrol the pump to deliver the drug or therapeutic agent with thedosage and frequency (or rate) specified by the one or more therapyprograms. As such, the techniques of this disclosure should not beconsidered limited to the example described in FIG. 9.

The techniques described in this disclosure may be implemented, at leastin part, in hardware, software, firmware or any combination thereof Forexample, various aspects of the techniques or components may beimplemented within one or more processors, including one or moremicroprocessors, digital signal processors (DSPs), application specificintegrated circuits (ASICs), field programmable gate arrays (FPGAs),programmable logic circuitry, or the like, either alone or in anysuitable combination. The term “processor” or “processing circuitry” maygenerally refer to any of the foregoing circuitry, alone or incombination with other circuitry, or any other equivalent circuitry.

Such hardware, software, or firmware may be implemented within the samedevice or within separate devices to support the various operations andfunctions described in this disclosure. In addition, any of thedescribed units, modules or components may be implemented together orseparately as discrete but interoperable logic devices. Depiction ofdifferent features as modules or units is intended to highlightdifferent functional aspects and does not necessarily imply that suchmodules or units must be realized by separate hardware or softwarecomponents. Rather, functionality associated with one or more modules orunits may be performed by separate hardware or software components, orintegrated within common or separate hardware or software components.

When implemented in software, the functionality ascribed to the systems,devices and techniques described in this disclosure may be embodied asinstructions on a computer-readable medium such as random access memory(RAM), read only memory (ROM), non-volatile RAM (NVRAM), electricallyerasable programmable ROM (EEPROM), Flash memory, and the like. Theinstructions may be executed by a processor to support one or moreaspects of the functionality described in this disclosure.

Various examples have been described. These examples, however, shouldnot be considered limiting of the techniques described in thisdisclosure. These and other examples are within the scope of thefollowing claims.

The invention claimed is:
 1. An implantable medical device comprising: ahousing that encloses at least a communication module; a first electrodeof a communication dipole electrically coupled to the communicationmodule; and an electrically conductive fixation mechanism that iselectrically coupled to a portion of the housing and that includes adielectric material that covers at least part of a surface of thefixation mechanism, wherein a portion of the fixation mechanism isconfigured to function as at least part of a second electrode of thecommunication dipole, and wherein the communication module is configuredto transmit or receive a modulated signal between the first electrodeand second electrode of the communication dipole.
 2. The implantablemedical device of claim 1, wherein the portion of the electricallyconductive fixation mechanism that is configured to function as at leastpart of the second electrode is not covered by the dielectric material.3. The implantable medical device of claim 1, wherein a portion of thehousing not electrically coupled to the electrically conductive fixationmechanism is configured to function as the first electrode.
 4. Theimplantable medical device of claim 3, wherein the portion of theelectrically conductive fixation mechanism that is configured tofunction as at least part of the second electrode is located at aposition on the fixation mechanism that is a further distance from thefirst electrode than any other portion of the housing.
 5. Theimplantable medical device of claim 4, wherein the portion of theelectrically conductive fixation mechanism that is configured tofunction as at least part of the second electrode is located at aposition on the fixation mechanism that is a furthest distance from thefirst electrode.
 6. The implantable medical device of claim 3, whereinan axis of the communication dipole between the portion of the housingconfigured to function as the first electrode of the communicationdipole and the portion of fixation mechanism configured to function asat least part of the second electrode of the communication dipole isoffset from a central axis of a vasculature when the implantable medicaldevice is positioned within the vasculature.
 7. The implantable medicaldevice of claim 6, wherein the axis of the communication dipole and thecentral axis of a vasculature when the implantable medical device ispositioned within the vasculature form an angle relative to one anotherat their point of intersection.
 8. The implantable medical device ofclaim 3, wherein a second portion of the housing that is electricallycoupled to the fixation mechanism is configured to function as part ofthe second electrode in conjunction with the portion of the electricallyconductive fixation mechanism.
 9. The implantable medical device ofclaim 8, further comprising a second electrically conductive fixationmechanism that is electrically coupled to the first portion of thehousing that is configured to function as the first electrode, wherein aportion of the second electrically conductive fixation mechanism isconfigured to function as at least part of a second electrode of thecommunication dipole.
 10. The implantable medical device of claim 1,wherein the portion of the housing electrically coupled to theelectrically conductive fixation mechanism serves as a ground plane forthe communication module.
 11. The implantable medical device of claim 1,wherein a thickness of the dielectric material covering the portion ofthe electrically conductive fixation mechanism that is configured tofunction as at least part of the second electrode is thinner than athickness of the dielectric material covering other portions of theelectrically conductive fixation mechanism.
 12. The implantable medicaldevice of claim 1, wherein the portion of the electrically conductivefixation mechanism that is configured to function as at least part ofthe second electrode is covered by a dielectric material that has ahigher dielectric constant than a dielectric constant of the dielectricmaterial covering other portions of the electrically conductive fixationmechanism.
 13. The implantable medical device of claim 1, wherein theportion of the electrically conductive fixation mechanism that isconfigured to function as at least part of the second electrode includesa fractal coating on a surface of the portion.
 14. The implantablemedical device of claim 13, wherein fractal coating comprises one of atitanium nitride coating, platinum oxide coating, iridium oxide coating,and carbon nanotube coating.
 15. The implantable medical device of claim1, further comprising a second electrically conductive mechanism that ismechanically coupled to the housing and electrically coupled to thecommunication module within the housing, wherein the second electricallyconductive mechanism includes a dielectric material that covers part ofa surface of the second electrically conductive mechanism, wherein aportion of the second electrically conductive mechanism is configured tofunction as at least part of the first electrode.
 16. The implantablemedical device of claim 15, wherein the portion of the secondelectrically conductive mechanism that is configured to function as atleast part of the first electrode is not covered by the dielectricmaterial.
 17. The implantable medical device of claim 16, wherein theportion of the electrically conductive fixation mechanism that isconfigured to function as at least part of the first electrode includesa fractal coating.
 18. The implantable medical device of claim 15,wherein the portion of the second electrically conductive mechanism thatis configured to function as at least part of the first electrode islocated at a position on the second conductive mechanism that is afurther distance from the second electrode than any portion of thehousing.
 19. The implantable medical device of claim 15, furthercomprising: at least a third electrically conductive mechanism that ismechanically coupled to the housing and electrically coupled to thecommunication module within the housing, wherein the third electricallyconductive mechanism includes a dielectric material that covers part ofa surface of the third conductive mechanism, wherein a portion of thethird electrically conductive mechanism is configured to function as atleast part of a third electrode a switching device electrically coupledbetween the communication module and the second and third conductivemechanisms, wherein the switching device selectively couples one of thesecond and third conductive mechanisms to the communication module. 20.The implantable medical device of claim 19, wherein the switching deviceselectively couples one of the second and third conductive mechanisms tothe communication module based on a signal quality of a signal receivedby the implantable medical device.
 21. The implantable medical device ofclaim 19, wherein the switching device selectively couples one of thesecond and third conductive mechanisms to the communication module inresponse to a command from an external device.
 22. The implantablemedical device of claim 19, wherein the switching device couples both ofthe second and third conductive mechanisms to the communication moduleconcurrently.
 23. The implantable medical device of claim 19, wherein asurface area of the portion of the second electrically conductivemechanism that is configured to function as at least part of the secondelectrode is different than a surface area of the portion of the thirdelectrically conductive mechanism that is configured to function as atleast part of the third electrode; and the switching device electricallycouples one of the second electrode and the third electrode to thecommunication module based on least in part on the surface areas. 24.The implantable medical device of claim 1, wherein the fixationmechanism is a cylindrical stent-like structure that is configured tolodge against a vessel wall.
 25. The implantable medical device of claim1, wherein the fixation mechanism includes at least two fixation loopsmechanically coupled to the housing.
 26. The implantable medical deviceof claim 25, wherein the at least two fixation loops are formed from asingle, integral piece.
 27. The implantable medical device of claim 1,further comprising a sensor within the housing to sense at least oneparameter of a patient